The present invention relates to an image pickup assembly and more particularly to a pickup tube for such a device in which the inside and outside diameters of the pickup tube are precisely formed so that the longitudinal mechanical axes defined by the midpoints of the respective inside and outside diameters of the tube coincide with the electron optical axis of the tube.
The operation of an image pickup tube requires the focusing and deflection of an electron beam. For this purpose the image pickup tube is provided with a coil assembly including a focusing coil and a deflection coil. The image pickup tube is centered in the coil assembly to form an image pickup assembly.
It is known that the geometrical distortion of an image on the image pickup tube is proportional to the square and the cube of its distance from the central axis of the deflection coil. The axis of the coil must be aligned with the axis of the image pickup tube and the optical axis within close tolerances. The optical axis of the camera and the axis of the coil are aligned with great precision but there is difficulty in aligning the pickup tube with the coil assembly because image pickup tubes are manufactured with glass envelopes having varying wall thicknesses and surface irregularities.
In a color television camera, three image pickup assemblies are usually utilized. If the camera is slightly shocked in handling or if replacement tubes are installed, misregistration, i.e. geometrical distortion in a multi-tubed color camera, will occur. This misregistration adversely affects the quality of the image produced by the color television camera and must be minimized as much as possible.
In U.S. Pat. No. 4,039,986 issued on Aug. 2, 1977 to Nakazawa et al., it is disclosed that misregistration should be suppressed to less than 100 microns. That is, the axis of the coil assembly must be aligned with the axis of the image pickup tube and with the optical axis to a precision of not more than 100 microns. In the first figure of the Nakazawa et al. patent, elastic material, such as gum rubber, is used at two points along the tube (not at the remote ends) for mounting the tube. As illustrated in the second figure of the Nakazawa et al. patent, the elastic material may be embedded in a holder. The elastic material utilized in that patent has a minimum inner diameter that is smaller than the outer diameter of the pickup tube and depends upon the elasticity to absorb the irregularity of the outer diameeter of the pickup tube. These elastic material pieces require a special shape and they are not only expensive to manufactuure but are also difficult to manufacture with high dimensional precision. This lack of precision results in difficulty of alignment of the central axis of the coil assembly with that of the image pickup tube. Further, these elastic members are known to wear and the restoring forces may not completely return, making alignment difficult when replacing a tube or when the camera is shocked in handling. Further, as discussed in the reference, the image pickup device is mounted using O-rings made of elastic material along the length of the tube. These O-rings have essentially the same problems as the other elastic materials. They are difficult to manufacture with high dimensional precision, are known to wear and the restoring forces may not completely return the tube when shocked in handling or when replacing a tube. An added disadvantage of prior solutions is that the tubes are not securely held to prevent axial motion with respect to the yoke. This axial motion will cause focus and alignment problems.
An alternative to the Nakazawa et al. structure is described in U.S. Pat. No. 4,191,936 issued on Mar. 4, 1980 to Colgan. The Colgan patent discloses a structure in which an image pickup tube is rigidly centered and mounted to a coil assembly by an adjustable split-ring clamp of hard plastic material mounted to the coil assembly. The split-ring clamp, when adjusted for mounting, compresses about the periphery of the faceplate or anti-halation disk extension of the tube to thereby provide a rigid mounting between the coil assembly and the tube. The rear portion of the coil assembly includes a yoke with a tapered inner surface. A split-ring wedge is forced between the tube and the rear portion of the yoke to rigidly support and center the rear portion of the tube.
Both the Colgan and the Nakazawa et al. patents disclose structures to retain an image pickup tube within a coil assembly in such a manner as to minimize the inherent eccentricity of the tube rather than to improve the concentricity of the tube itself.